1. Field of the Invention
The invention relates to a technology of detecting degradation of a NOX sensor disposed downstream of an exhaust gas control apparatus that includes a three-way catalyst and a selective reduction catalyst.
2. Description of the Related Art
As a system that purifies exhaust gas of a lean-burn internal combustion engine, there is known a system in which a three-way catalyst, a storage reduction NOX catalyst and a selective reduction catalyst are disposed in that order from the upstream side (e.g., see Japanese Patent Application Publication No. 2008-303759 (JP-A-2008-303759)).
In the foregoing construction, it is conceivable to adopt a construction in which a NOX sensor is disposed in an exhaust passageway downstream of the selective reduction catalyst for the purpose of performing a feedback control of the air/fuel ratio of exhaust gas in order to keep the amount of nitrogen oxides (NOX) discharged from the selective reduction catalyst to or below a target value, or detecting the degree of degradation of the selective reduction catalyst, or the like. In such a construction, it is also important to detect or determine degradation of the NOX sensor.